1. Field of the Invention
The present invention relates generally to a method and apparatus for generating a stable oscillation signal over a wide frequency range and, more particularly, to a wide range voltage controlled oscillator and a phase locked loop using such an oscillator.
2. Description of the Related Art
A phase locked loop is a feedback circuit used to generate an output signal whose frequency tracks any variation in the frequency of an input signal. FIG. 1 shows a conventional phase locked loop circuit. In the phase locked loop of FIG. 1, a phase detector 2 detects a phase difference PD between an input reference signal and an oscillation signal output by a voltage controlled oscillator 6. The phase difference PD functions as an error feedback signal, varying with the difference in frequency between the input reference signal and the oscillation signal output. A low-pass filter 4 filters the phase difference signal PD and outputs a voltage control signal VC. The voltage controlled oscillator 6 receives the voltage control signal VC and generates an oscillation signal having a frequency which varies according to the oscillating control signal VC. The frequency of the oscillation signal output is determined by the magnitude of the voltage control signal VC, which in turn is determined by the magnitude of the phase difference PD. Thus, the frequency of the oscillation signal is adjusted according to the error between the input reference signal and the oscillation signal, resulting in the oscillation signal frequency tracking the input reference signal frequency.
This conventional type of phase locked loop has been used in many applications, such as an FM demodulator, a frequency synthesizer, a digital data recording and reproducing apparatus, etc. Different applications may require a phase locked loop in which the reference signal input to the phase detector has a frequency varying within a narrow frequency range or within a wide frequency range. In a phase locked loop which receives a reference signal having a wide frequency range (called a wide range phase locked loop), it is preferable that the sensitivity, and thus the gain, of the voltage control oscillator is very high.
However, when the gain of the voltage controlled oscillator is very high, the voltage controlled oscillator and the phase locked loop become sensitive to noise, and undesirable variations in the frequency of the oscillation signal occur, producing jitter. For example, a voltage controlled oscillator with an output range of 1 to 300 MHz which receives an input reference signal varying within a range of 3V has a gain of 100 MHz/V (or 100 kHz/mV). A voltage controlled oscillator having such a large gain becomes sensitive to noise, to such an extent that its jitter characteristics become a serious problem. The present invention is directed to overcoming or at least reducing the effects of this problem.